Optical lithography is the predominate process for making integrated circuits. Integrated circuits are built as a series of layers, sometimes 15 or more. To form each layer of an integrated circuit, the wafer is coated with a photosensitive polymer, called a photoresist. The layer of photoresist is exposed to a pattern of light. Depending on the type of photoresist, certain areas of the photoresist are soluable following exposure and are rinsed with a solvent. The remaining photoresist, which is insoluable, delineates a pattern on the surface of the wafer that is protected from the next processing step, which could be, for example, etching, dielectric or metal deposition, or ion implantation. The regions delineated by the boundaries of the remaining photoresist correspond to physical elements, such as parts of transistors and connections between them, to be reproduced on each integrated circuit.
To form the pattern of light on the wafer, an “illuminator” is used to create a coherent, uniform beam of light that is projected through a photomask (or “mask”) or reticle. A mask is typically a plate made of quartz or other translucent material, and the pattern on the mask is formed with chrome or other opaque material disposed on the surface on the plate. This chrome layer may also be covered with an anti-reflective layer of chrome oxide. A lens system collects the light transmitted through the mask and focuses it on a small area of a semiconductor wafer, thereby creating a scaled-down image on the photoresist. The exposure is repeated across and down the wafer by incrementally moving the wafer using a “stepper” machine, thereby an array of side-by-side images on the photoresist covering the wafer.
As the size of features of integrated circuits shrink, the wavelength of light illuminating the masks must also decrease. However, quartz lenses that are used in the illuminator and lens systems absorb wavelengths of light shorter than 193 nm and therefore cannot be used for smaller wavelengths of light. Although equipment has been proposed that is capable of using a shorter wavelength of light, there exist several resolution enhancement techniques that permit the same wavelength of light to resolve smaller features. For example, using known resolution enhancement techniques, 193 nm light can be used to resolve features as small as 100 nm or less.
One of type of resolution enhancement technique is to improve contrast between the light and dark areas of the pattern projected onto the wafer using destructive interference. Phase shifting may be accomplished several different ways. One is by very precisely etching part of the substrate. Such a mask is sometimes referred to as a Levenson phase shift mask (PSM) or alternating aperture PSM (AAPSM). A different approach is the use of an embedded layer that both shifts the phase of the light and attenuates it. This type of mask is called an attenuating phase shift mask (EAPSM). It too sets up destructive interference along the border of a feature region. However, unlike a AAPSM, an EAPSM is considered to be a weak phase shifter due to relatively high degree of attenuation of the light. Typically only of 6% of the incident radiation is transmitted. An opaque film of material—for example a material comprised or based of molybdenum silicide (MoSi)—is applied to the substrate. This material shifts phase of the light that passes through it by 180 degrees and substantially attenuates it. The resulting image becomes sharper at the feature region borders as a result of the small amount of 180 degree phase shifted light destructively interfering with the unshifted light passing through the adjacent opening. The transmission and phase shift provided by the phase shift layer depend on radiation wavelength and the intrinsic properties of the phase shift layer material (e.g., its absorption coefficient and index of refraction). Thus, for a particular radiation wavelength, the transmission and phase shift of the radiation incident on the phase shift layer of a mask become a function of the thickness of the phase shift layer.